Horses tolerate heat poorly — worse than people do. A large body produces a lot of heat and gets rid of it mainly through sweating, and that very system jams up on hot, humid days: during hard work in the heat, a horse can easily lose ten liters of fluid an hour. Heat stress occurs when heat production outpaces heat loss, and it can escalate into life-threatening overheating. This guide covers recognizing it, acting immediately, and — better still — preventing it.
When the Risk Rises
Three factors stack up: temperature, humidity, and workload. A dry day of 30°C with a quiet walk is a different story than 26°C with muggy air and a jumping session. Rule of thumb: the more humid the air, the less sweat evaporates and the worse the horse's cooling system works. Extra risk applies to overweight and insufficiently fit horses, horses with a long coat (think of older horses with Cushing's), dark-colored horses in full sun, and horses being transported in hot weather.
The Signs
- Rapid breathing that doesn't settle — if the horse is still panting or flaring its nostrils even after minutes of rest, it isn't shedding its heat.
- Persistent heavy sweating — or, conversely, dry skin when conditions call for sweating (no longer being able to sweat is called anhidrosis and is a warning sign).
- Lethargy and apathy: slow reactions, a hanging head, no interest in the surroundings.
- Body temperature above 39.5°C that doesn't come down with rest — normal is 37 to 38°C. A thermometer belongs in the first-aid kit.
- In severe cases: stumbling, muscle trembling, dark mucous membranes.
What to Do Immediately
- Stop the work and find shade, ideally with a breeze or a fan.
- Cool with plenty of cold water over the whole body — bucket after bucket, or a hose, including over the large muscle groups, and keep repeating. Research around the hot championships of recent years is clear on this: continuously pouring on cold water is the most effective cooling method, and the old advice that you should keep scraping the water off, or that cold water causes muscle problems, isn't true.
- Let the horse drink. That old prohibition is outdated too: a hot horse is allowed to drink water.
- Check the temperature every ten minutes. If it doesn't drop toward normal, or stumbling and apathy appear, call the vet immediately. Overheating is an emergency.
Prevention: Plan Around the Heat
- Ride during the cool hours — early morning or late evening — and scale back intensity; a heat wave is not the week for interval training.
- Build fitness before summer arrives. A fit horse produces less heat for the same work; see building fitness.
- Provide shade and fresh water in the pasture, and consider turnout at night instead of during the day in hot spells.
- Replenish what's sweated out. With days of heavy sweating, a horse loses salts along with fluid; a salt lick or electrolyte supplement plus unlimited water keep the balance in check.
- Plan transport and competitions deliberately: leave early, keep ventilation open, never let a horse wait in a stationary trailer in the sun.
- Rinse off after training and don't unsaddle in full sun.
In Short
- Heat stress = heat production exceeds heat loss; humid heat is more dangerous than dry heat, and work is the biggest driver.
- Recognize it by breathing that doesn't settle, persistent sweating or, conversely, no sweating at all, lethargy, and a temperature above 39.5°C.
- First aid: shade, plenty of cold water over the whole body and keep repeating, let it drink; no recovery means the vet — it's an emergency.
- Prevention is about planning: cool hours, adjusted intensity, shade and water, electrolytes with heavy sweating, thoughtful transport.
See also: Transporting a Horse in Hot Weather · When to Call the Vet · The First-Aid Kit · Turnout and Outdoor Living · Jargon: heat stress · Level: Considering Your Own Horse